The present invention is directed to a method and apparatus for the improved characterization of a digital image input terminal such as a scanner or digital camera, so as to enable the device to be employed in the measurement and analysis of color images. For the purpose of describing the invention, a scanner will be used as an exemplary device. The invention uses a family of scanner characterization targets, each varying in primary colorants and at a fixed level of black (K) or luminance. A corresponding family of scanner characterizations is derived, one for each level of K, and the transformation are prepared for each value of K, such that the characterization includes K as an input—thereby improving the characterization and accuracy of the scanner. The family of targets may be combined into one physical target; and similarly, the family of scanner characterizations may be combined into a single characterization that accepts 4-dimensional input.
Heretofore, a number of patents and publications have disclosed the measurement of characteristics of color images and color image output devices (image output terminals (IOT)), the following patents and publication are hereby incorporated by reference for their teachings, and the relevant portions of which are briefly summarized as follows:
U.S. Pat. No. 6,529,616, for a “TECHNIQUE FOR ACCURATE COLOR—COLOR REGISTRATION MEASUREMENTS,” by Rasmussen, et al., issued Mar. 4, 2003, discloses a test pattern and measurement technique used to allow highly accurate measurements of color-color registration in an image output device that prints in cyan, magenta, yellow, and black. The technique automatically factors out errors originating from skew between the detector and the subject of measurement;
U.S. Pat. No. 6,522,430 directed to “QUANTIFICATION OF MOTION QUALITY EFFECT ON IMAGE QUALITY,” by Dalal et al., issued Feb. 18, 2003, teaches a special test pattern and measurement technique used to allow highly accurate measurements of motion quality defects in an image output device that prints in monochrome or color;
U.S. Pat. No. 6,275,600, to Banker et al., for “MEASURING IMAGE CHARACTERISTICS OF OUTPUT FROM A DIGITAL PRINTER,” issued Aug. 14, 2001, teaches a method for measuring image characteristics of printed output from a digital printer by sending test pattern data to the digital printer, generating a printed image of the test pattern data at the digital printer, scanning the printed image to obtain digital pattern data, and analyzing the digital pattern data of the printed image. The test pattern that is printed includes target objects designed to reveal specific printed image characteristics, and the analysis of the data from scanning the printed image includes the generation of one or more quantitative ratings with respect to printed image characteristics.
“Refinement of Printer Transformations Using Weighted Regression,” by R. Balasubramanian and M. Maltz, SPIE, August 1996, Vol. 2658 (p. 334-340), which is directed to the use of transformations in printer characterization and color correction, and proposes a method for improving the accuracy of such transforms that uses least-squares regression.
Since color scanners and similar image input terminals are commonly available in many imaging environments, it would be of significant benefit if such scanners could be used as color measurement devices for characterizing output devices such as hardcopy printers and the like. Furthermore, because of the high spatial resolution available with most scanners today, they can also be very effective in diagnosing problems with hardcopy devices if they can be accurately characterized to physically measurable color. Examples include scanner-based printer calibration and characterization and printer diagnostics. However, with standard color management, typical scanners provide only mediocre colorimetric accuracy limiting their use in these applications.
Traditional approaches for scanner color management create a characterization profile by scanning a printed target containing color patches. The target is simultaneously measured with a color measurement device to obtain spectral reflectance or colorimetric measurements such as CIELAB. Scanner characterization is the process of relating the scanned (usually RGB) signals to the spectral or colorimetric representation. This process must generally be repeated for each input medium (i.e. combination of substrate, colorants, and image path elements). Thus different scanner color characterization profiles are required for use with a photograph and an electrophotographic print. The primary reason for this is that color scanners are not calorimetric, so that the relationship between the response of the scanner and that of the human eye depends heavily on the spectral properties of the medium being scanned.
For printed media comprising four or more colorants (e.g. cyan, magenta, yellow and black (CMYK)), there is an added dependency on the particular colorant combinations being scanned. The basic subtractive primaries in printing are cyan (C), magenta (M), and yellow (Y). However many marking processes, e.g. lithography, electrophotography, and inkjet, use additional colorants. The most common colorant, and the one used herein to illustrate this invention, is black (K). Other colorant examples include “hi-fidelity” colorants such as orange and green. Whenever four or more colorants are used, there is an inherent redundancy, in that different colorant combinations can result in the same three-dimensional (3-D) response from either the human eye or the scanner. Thus the digital image input terminal or scanner characterization function depends not only on the physical properties of the medium, but also on the particular colorant combinations being scanned. The experiments described by G. Sharma, S. Wang, D. Sidavanahalli, and K. T. Knox, in “The impact of UCR on scanner calibration,” in Final Prog. and Proc. IS&T's PICS Conference, Portland, Oreg., 17-20 May 1998, pp. 121-124, illustrate this fact by demonstrating that the scanner characterization, even for a single printer, shows significant variation with the chosen undercolor removal (UCR) and gray component replacement (GCR) strategy.
Standard approaches to scanner characterization make fixed, a priori assumptions about the colorant combinations. Take as an example, the standard Kodak® Q60 lithographic target generated with CMYK combinations. The latter are formed via a predetermined UCR/GCR strategy (designed to suit a typical offset lithographic press), which constrains the amount of black (K) used with a given CMY combination. This approach is justified by the assumption that the lithographic pictorial images that one expects to scan in the final application are also subject to the same or similar UCR/GCR constraints.
A problem arises, however, when CMYK images are encountered that deviate from the assumed UCR/GCR strategy. A typical example would be in a print diagnostics application, where the images that are scanned are not pictorial images, rather they are test patterns designed for diagnosing print defects. In order to obtain visually relevant metrics for the defects, the scanned images are processed through a device characterization mapping RGB color space to CIELAB color space. However, the CMYK used to create the test prints are not necessarily subject to the UCR/GCR constraints normally used for pictorial images. For example, in order to test defects in the K channel, it may be necessary to scan a target containing a large patch of 50% K. Examining an interaction between C and K may require a test pattern of 50% C and 50% K. Typical UCR/GCR strategies optimized for pictures do not account accurately for such colorant combinations. Thus the use of a scanner characterization optimized for a fixed UCR/GCR strategy may give erroneous results. Sharma et al., as noted above, report that significant errors can arise from incorrect assumptions of UCR/GCR in scanner characterization. Many of the larger errors occur in the dark region (which is not surprising since this is where K is usually heavily involved). For the print diagnostic application, such errors may be unacceptable.
Another application where it may be desirable to use the scanner as a color measurement device is printer calibration and characterization. Printer calibration and characterization involves printing and measuring targets comprising patches of various (preferably unconstrained) CMYK combinations, and modeling the printer's response throughout its gamut. In this application one cannot generally assume that the target being scanned has been generated with a fixed UCR/GCR strategy.
Take gray-balance calibration as a specific example. Such a calibration involves searching various CMY combinations in the vicinity of the device neutral (C=M=Y) axis from 0% to almost 100% area coverage, and finding those combinations that match a certain aim (e.g. a*=b*=0; or a* and b* values that match those of the K channel). Purely three-colorant neutrals are not encountered in typical UCR/GCR strategies, as these usually introduce K for the darker grays. As a second example, calibration of the K channel requires measurement of pure K patches from 0% to 100% area coverage. Again these are not encountered in standard UCR/GCR strategies. Thus a standard scanner characterization designed for capturing pictorial images generated with a standard UCR/GCR, would not be “trained” to accurately measure such patches. Similar examples apply for printer characterization.
To effectively use a digital image input terminal as a color measurement device for printer calibration, characterization or diagnostics applications thus requires a device characterization that can accurately describe the printed color regardless of the underlying colorant combination. The invention disclosed herein is directed to a method to achieve device (e.g., scanner) characterization meeting these requirements. The present invention is, therefore, directed at a novel device characterization system and methodology for applications requiring color measurement of printed hardcopy output in diagnostic and calibration and characterization applications. The invention overcomes the limitation of standard scanner color characterization techniques and greatly improves the accuracy in these specialized applications. It is therefore a significant enabler for use of scanners and other image input terminals as color measurement devices in these applications.
In accordance with the present invention, there is provided a method for characterizing a digital image input terminal for color measurement of printed hardcopy, comprising the steps of: printing, for each of a plurality of levels of K, a target comprising a grid of patches having varying levels of C, M, and Y; measuring the colors of patches in said grid of patches in a first color space; scanning the grid of patches on the digital image input terminal to generate a representation of the patches in an input device dependent color space; and deriving, for each of a plurality of levels of K, a characterization transform mapping the scanned device dependent color space values to the first color space.
In accordance with another aspect of the present invention, there is provided a method of performing color image rendering quality analysis on an image output device having an output station that generates a hardcopy color image output from an input image, the method comprising: characterizing an image input terminal, such as a scanner, for color measurement of printed hardcopy, including printing, for each of a plurality of levels of K, a target comprising a grid of patches having varying levels of C, M, and Y, measuring the color of the patches in the grid in a first color space, scanning the patches on a digital image input terminal to generate a representation of the patches in input device color space, and deriving, for each level of K, a scanner characterization transform mapping the scanned input device color space values (e.g., RGB) to the first color space; generating a hardcopy image output from the image output device using a digital test pattern as an input, said digital test pattern including a plurality of color patches, and associated black-only patches representing the value of K for the color patches; scanning the hardcopy image output from the image output device with the scanner to form a digital image; performing a color space transformation for at least one of the plurality of colors on the hardcopy image output from the image output device to produce a color space value for the patch in the first color space; and performing a quality analysis on the hardcopy output based on a comparison of the accurately characterized color patch in the first color space in relation to an intended color for that patch.
In accordance with yet another aspect of the present invention, there is provided a system for performing color image quality analysis on an image output device having an output station that generates a hardcopy color image output in response to an input image, comprising: a test target comprising a grid of patches having varying levels of C, M, and Y at a plurality of levels of K; a scanning system including a processor, memory and scanner in communication with the processor and memory, wherein the scanner is characterized for color measurement of printed hardcopy by scanning the patches on the test target to generate a representation of the patches in RGB color space, and in conjunction with measured color data for the test target patches, in a first color space, a scanner characterization transform is generated and stored in the memory as a mapping from the device dependent color space to the first color space; a hardcopy image, generated from the image output device in response to a digital test pattern input, said digital test pattern including a plurality of color patches and associated black-only patches representing the value of K for the color patches, wherein the hardcopy image is scanned with the scanning system to produce digital image data, and the digital image data for at least one of the color patches is transformed, by the scanning system, to produce a color space value for the patch in the first color space; and means for performing a quality analysis on the hardcopy output based on a comparison of the accurately characterized color patch in the first color space in relation to an intended color for that patch as specified by the digital test pattern.
One aspect of the invention deals with a basic problem in the use of digital image input terminals for colorimetric or similar image analysis functions—characterization of the scanner. This aspect is further based on the discovery of a technique that alleviates this problem. The technique uses a family of device characterization targets, each varying in primary colorants and at a fixed level of black (K) or luminance. A corresponding family of device characterizations is derived, one for each level of K, and the transformation are prepared for each value of K, such that the characterization includes K as an input—thereby improving the characterization and accuracy of the input device. As a result of the invention, conventional image input terminals such as scanners may be employed to more accurately measure and analyze output from hardcopy image output devices.
The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.